“High voltages” means a voltage level of at least 10 kilovolts (kV), but often much higher, such as hundreds of kV. This voltage has to be taken by the insulating layer, since the conductor of the cable is on high voltage potential and the periphery of the cable has to be on earth potential, and said insulating layer is for that sake normally surrounded by a semiconducting thin shielding layer. This causes dielectric stress upon the insulating layer, which has to be dimensioned for reliably taking this stress.
Furthermore, when transmitting electric power through High Voltage Direct Current (HVDC) the losses are reduced when the voltage is increased, so that it is for that reason a desire to increase said voltage.
For illuminating, but not in any way restricting the present invention, the use of a cable of this type for HVDC transmission is very schematically illustrated in FIG. 1. A plant for transmitting electric power shown there has a direct voltage network 1 for HVDC having two said cables 2, 3 for interconnecting two stations 4, 5, which are configured to transmit electric power between the direct voltage network 1 and an alternating voltage network 6, 7 here having three phases and connected to the respective station. One 2 of the cables is intended to be on positive potential of half the direct voltage of the direct voltage network, while the other cable 3 is on negative potential of half of the direct voltage. Accordingly, this plant has a bipolar direct voltage network, but a monopolar network with a return current flowing through earth electrodes is also conceivable.
There is a need for transmitting more power than possible today in HVDC transmissions, but cables for higher power than 800 MW are still not developed. Should this be done without increasing the dimensions of the cable, which already today are impressive and close to transport limits, either the current has to be increased by conductors with higher conductivity or the voltage has to be increased by higher stress to said insulating layer. The conductivity of the conductor is limited by the conductor material, copper and aluminium, which cannot be improved and other conductors are not available within the foreseeable future or are far too expensive (superconductors) for constituting any real option. Thus, the other way to increase the power in such transmissions is by improving the insulating material, which seems to be the most promising way to substantially increase the power and is also favourable owing to the reduction of losses obtained by increasing the voltage.
There are two known types of HVDC cables, mass impregnated cables (thick insulating layer normally formed by a paper impregnated by oil) and extruded cables (insulating layers on polymer base). The average electric field acceptable for these cables (for the mass impregnated cables) is around 30 kV per millimeter and for the extruded cables around 20 kV per millimeter. The mass impregnated cables may be improved by exchanging some or all of the paper by a plastic film, but that would make the impregnation more difficult. Moreover, the extruded cables have probably still potential to have increased field by utilising improved materials, in which one goal is to double the dielectric stress to 40 kV per millimeter. Appended FIG. 2 shows a known extruded cable having an inner conductor 8 surrounded by a thin semiconducting layer 9 having potential equalizing properties, a thick insulating layer 10 of polymer base, such as cross-linked polyethylene outside thereof and an outer thin semiconducting shielding layer 11 also being potential equalizing. Such a cable is also known through EP 0 868 002.
U.S. Pat. No. 6,509,527 discloses a use of a cable insulating layer making it possible to increase the dielectric stress to a cable of this type.
Both technologies described above for producing a DC cable have a design criteria that dielectric faults shall not occur during the lifetime of the cable, which is 40 years. This puts very stringent requirements on reliability of the design and the voltage stress has to be much lower than it would be if more frequent failures were accepted.